1. Field of the Invention
This invention relates to a method and apparatus for acquiring information concerning a bone constituent in a bone of a human body, or the like. This invention particularly relates to a bone image processing method and apparatus for acquiring numerical information concerning the density of a bone constituent and image information representing the form of bone trabeculae, the information being useful in making a diagnosis of osteoporosis, or the like. This invention also relates to a bone image processing method and apparatus for acquiring information representing a change of bone trabeculae of an object from temporally different image signals representing images of the object.
2. Description of the Prior Art
Bone mineral analysis, i.e., quantitative determination of amounts of calcium in bones, is useful for making a diagnosis for preventing fractures of bones.
Specifically, the amounts of the bone mineral are determined by the density of bone trabeculae, which are the cancellous matter constituting the internal regions of bones, i.e. the bone density. Therefore, if the bone density is low, the image density of a bone pattern in a bone image will become high. If the bone density is high, the image density of the bone pattern in the bone image will become low.
Therefore, by investigating small changes in the amounts of calcium contained in bones, osteoporosis can be found early, and fractures of the bones can be prevented.
Various techniques for bone mineral analysis have been proposed and used in practice. Such techniques include microdensitometry (MD technique), single photon absorptiometry (SPA technique), dual photon absorptiometry (DPA technique), quantitative digited radiography (QDR technique), quantitative computer tomography (QCT technique), and dual energy quantitative computer tomography (DQCT technique).
However, the aforesaid techniques have the drawbacks in that amounts of the bone mineral in vertebral bones, which directly indicate the sign of an osteoporosis, cannot be determined, in that a large-scaled apparatus must be used, or in that the radiation dose cannot be kept small.
Accordingly, in U.S. Pat. No. 5,122,664, the applicant proposed a novel method for quantitatively analyzing a bone mineral, wherein energy subtraction processing is employed.
Specifically, the applicant proposed a method for quantitatively analyzing a bone mineral by carrying out energy subtraction processing wherein each of at least two recording media (such as stimulable phosphor sheets or sheets of X-ray film) is exposed to one of at least two kinds of radiation, which have different energy distributions and carry image information of an object comprising a bone tissue and a soft tissue, radiation images of the object are thereby recorded on the recording media, a digital image signal made up of a series of image signal components representing each radiation image is detected from each recording medium, the image signal components of the digital image signals thus obtained, which image signal components represent corresponding picture elements in the radiation images, are then subtracted from each other such that the soft tissue pattern may be erased, and a difference signal is thereby obtained which represents the image of only the bone tissue (a bone image). The proposed method for quantitatively analyzing a bone mineral comprises the steps of:
i) recording a pattern of a bone mineral reference material, which simulates bones of a human body and in which known amounts of the bone mineral vary step-wise, together with the pattern of the object when each of the radiation images of the object is recorded on each of the recording media, and PA1 ii) quantitatively analyzing the amount of the bone mineral by comparing the image density of the pattern of the bone tissue and the image density of the pattern of the bone mineral reference material with each other, both patterns appearing in the bone image. PA1 i) carrying out the energy subtraction processing on the two radiation images, a bone image, in which the pattern of the bone tissue in the object has been extracted or emphasized, being thereby formed, PA1 ii) obtaining numerical information concerning the density of the bone constituent in accordance with the bone image, and PA1 iii) carrying out a bone trabecula pattern emphasizing process on the bone image, information representing the form of bone trabeculae being thereby obtained. PA1 i) carrying out the energy subtraction processing on the two radiation images, a bone image, in which the pattern of the bone tissue in the object has been extracted or emphasized, being thereby formed, PA1 ii) obtaining numerical information concerning the density of the bone constituent in accordance with the bone image, and PA1 iii) carrying out a bone trabecula pattern emphasizing process on at least either one of the two radiation images or on an addition image represented by an addition image signal, information representing the form of bone trabeculae being thereby obtained, the addition image signal being obtained by weighting image signal components of each of radiation image signals representing the two radiation images and by adding the weighted image signal components of the radiation image signals to each other, which image signal components represent corresponding picture elements in the two radiation images. PA1 forming the two radiation images of the single object such that a pattern of a bone mineral reference material, in which the amounts of a bone mineral are known, (i.e., a bone mineral reference material, which simulates bones of a human body and in which known amounts of the bone mineral vary step-wise) may be embedded together with the pattern of the object in each of the two radiation images, PA1 forming an image density-to-bone mineral amount conversion table from the image density of the pattern of the bone mineral reference material, which pattern appears in the bone image, and the known amounts of the bone mineral, and PA1 converting the image density of the pattern of the bone tissue, which pattern appears in the bone image of the object, to the amount of the bone mineral in accordance with the image density-to-bone mineral amount conversion table. PA1 i) an energy subtraction processing means for carrying out the energy subtraction processing on two predetermined image signals, a bone image signal representing a bone image, in which a desired bone tissue pattern embedded in the images represented by the image signals has been extracted or emphasized, being thereby obtained, PA1 ii) a bone density analyzing means for obtaining numerical information concerning the density of the bone constituent in accordance with the bone image signal, and PA1 iii) a bone trabecula pattern emphasizing means for carrying out a bone trabecula pattern emphasizing process on at least either one of the radiation image signals representing the two radiation images before being subjected to the energy subtraction processing, on an addition image signal representing an addition image, or on the bone image signal having been obtained from the energy subtraction processing, the addition image signal being obtained by weighting image signal components of each of the radiation image signals representing the two radiation images and by adding the weighted image signal components of the radiation image signals to each other, which image signal components represent corresponding picture elements in the two radiation images. PA1 the bone density analyzing means may obtain the numerical information concerning the density of the bone constituent by: PA1 i) reading image signals representing two images, which are to be used for comparison, from an image storage means for storing image signals representing a plurality of images of a single object, which comprises a soft tissue and a bone tissue, the plurality of the images of the single object having been recorded at different points of time, PA1 ii) carrying out a bone trabecula pattern emphasizing process on each of the image signals having been read from the image storage means, the pattern of the bone trabeculae in the bone tissue being thereby emphasized, PA1 iii) carrying out a position matching process on the emphasized image signals, which have been obtained from the bone trabecula pattern emphasizing process, the positions of the two images represented by the emphasized image signals being thereby matched with each other, and PA1 iv) carrying out a subtraction process for subtracting the image signal components of the two emphasized image signals, which have been obtained from the position matching process, from each other, the image signal components representing corresponding picture elements in the two images, a difference signal representing a change of the bone trabeculae with the passage of time being thereby obtained. PA1 the position matching process and the subtraction process may be carried out on the image signals, which have been obtained from the smoothing process. PA1 i) a storage means for storing image signals representing a plurality of images of a single object, which comprises a soft tissue and a bone tissue, the plurality of the images of the single object having been recorded at different points of time, PA1 ii) a reading means for reading image signals representing two images, which are to be used for comparison, from the storage means, PA1 iii) a bone trabecula pattern emphasizing means for carrying out a bone trabecula pattern emphasizing process on each of the image signals having been read from the storage means, and thereby emphasizing the pattern of the bone trabeculae in the bone tissue, and PA1 iv) an image processing means for:
With the method proposed in U.S. Pat. No. 5,122,664, the density of the bone constituent can be determined accurately. However, there is a strong demand for a technique for furnishing information more useful in making a diagnosis of osteoporosis.
Specifically, even if the numerical information concerning the bone density alone is presented, it will be difficult for the condition of bone trabeculae to be ascertained.